1. FRACTURE and TOUGHNESS DESIGN
11/21/ /21/2018
FRACTURE and TOUGHNESS DESIGN
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2. Strength Toughness Resistance of a material to plastic flow
Resistance of a material to the propagation of a crack

3. This type of test provides a comparison of the toughness of materials
However, it does not provide a way to express toughness as a material property

4. Remote stress applied to a cracked material
The local stress is proportional to the number of lines of force which rises steeply as the crack tip is approach

5. Cracks concentrate stress
At the tip of a crack the stress can be very high:
Metals: the zone near the crack tip is plastic
Ceramics: the zone near the crack tip has micro-cracks
Composites: near the crack tip, delamination and debonding

6. Cracks propagate when the stress intensity factor exceeds a critical value
The critical value is known as the fracture toughness K1c

7. A plastic zone forms at the crack tip where the stress would otherwise exceed the yield strength

8. A material transitions from yield to fracture at a critical crack length

9. Critical crack lengths are a measure of the damage tolerance of a material
Tough metals are able to contain large cracks but still yield in a predictable, ductile manner

10. Values range from 0.01 – 100 MPa√m
Contours show the toughness, Gc

11. Transition crack length plotted on chart
Values can range from near-atomic dimensions for ceramics to almost a meter for ductile metals

12. Characteristic of ceramics and glasses
Local stress rises as 1/√r toward the crack tip
If it exceeds that required to break inter-atomic bonds, they separate, giving a cleavage fracture

13. Materials contain inclusions which act as stress concentrations when loaded
The inclusions separate from the matrix causing voids to nucleate and grow, causing fracture

14. If a material is ductile, a plastic zone forms at the crack tip
Within the plastic zone, voids nucleate, join, and link to cause fracture
The plasticity blunts the crack tip, reducing the severity of the stress concentration

15. At low temperatures some metals and all polymers become brittle
As temperatures decrease, yield strengths of most materials increase, leading to a reduction in the plastic zone size
Only metals with an FCC structure remain ductile at the lowest temperature

16. Impurities in an alloy are normally found in grain boundaries
This leads to a network of low-toughness paths that can lead to brittle fracture

17. Increasing the yield strength of a metal decreases the size of the plastic zone surrounding a crack
This leads to decreased toughness

18. Fillers, impact modifiers, and fiber reinforcement can significantly alter the fracture toughness of polymers

19. When a crack grows in a matrix, the fibers remain intact and bridge the crack

20. SUMMARY Toughness is resistance to the propagation of a crack
Tough materials yield rather than fracture
Toughness is a material property
By choosing materials with sufficient toughness, design can be immune to small cracks
Many metals and some composites are tough “enough” for this kind of design
Polymers and ceramics are not tough enough in most cases